I. Field of the Invention
The present invention relates to a tape drive control apparatus for a magnetic recording/reproducing apparatus using a magnetic tape as a recording medium.
II. Description of the Prior Art
FIG. 1 shows the overall arrangement of a tape drive system of a cassette video tape recorder (to be referred to as a VTR hereinafter). Referring to FIG. 1, reference numeral 1 denotes a tape cassette; and 2, a magnetic tape. Reference numerals 3 and 4 denote a supply reel shaft and a take-up reel shaft, respectively. The supply reel shaft 3 and the take-up reel shaft 4 can engage with a supply reel 5 and a take-up reel 6 of the tape cassette 1, respectively. Reference numerals 7 and 8 respectively denote a supply reel motor and a take-up reel motor which directly drive the supply and take-up reel shafts 3 and 4, respectively. Reference numeral 9 denotes a rotary head cylinder provided with a pair of rotary video heads. The magnetic tape 2 is wound around the rotary head cylinder 9 for an angular interval corresponding to an angle of about 180.degree.. The rotary head cylinder 9 performs helical scanning such that video signals are recorded to form oblique recording tracks on the magnetic tape 2 along its longitudinal direction, and such that the recorded video signals may be reproduced from the oblique recording tracks. A capstan 10 clamps the magnetic tape 2 with a pinch roller 11 so as to drive the magnetic tape 2 at a predetermined speed. Reference numeral 12 denotes a full-width erase head; 13, an audio erase head; and 14, an audio control head. In the fast forward and rewind modes, the capstan 10 is separated from the pinch roller 11, so that the magnetic tape 2 travels along a travel path 15.
FIG. 2 is a block diagram of a conventional tape drive control apparatus for controlling fast forward and constant-speed tape travel. Referring to FIG. 2, when the tape is fast forwarded by the reel motors without using the capstan 10, frequency generators (to be referred to as FGs hereinafter) 21 and 22 detect rotational velocity or rpm data of the supply and take-up reel motors 7 and 8, respectively. Output signals from the FGs 21 and 22 are supplied to f-v (frequency-voltage) converters 23 and 24, respectively. Voltage signals from the f-v converters 23 and 24 are mixed by a mixer 25 at a ratio of 1:1. A composite output from the mixer 25 is supplied to a take-up speed comparator 26 and is compared with a reference voltage generated from a reference voltage generator 27. An output from the take-up speed comparator 26 is supplied to a take-up reel motor driver 29 through a switch (to be referred to as an SW hereinafter) 28, so that the take-up reel motor 8 is controlled to take up the magnetic tape 2 at a substantially constant speed. On the other hand, an output from a supply reel motor torque generator 30 is supplied to a supply reel motor driver 32 through an SW 31 so that a predetermined torque is generated from the supply reel motor 7. As a result, a proper tension occurs in the magnetic tape 2.
When the magnetic tape is driven at a constant speed while it is brought into tight contact with the capstan 10 and the pinch roller 11, an output from a supply reel motor torque generator 33 is supplied to the supply reel motor driver 32 through the SW 31 so that a predetermined torque is generated from the supply reel motor 7 to apply a proper tension to the magnetic tape 2. At the same time, an output from a take-up reel motor torque generator 34 is supplied to the take-up reel motor driver 29 through the SW 28 so that the predetermined torque is generated from the take-up reel motor 8. Then, the magnetic tape 2 fed in tight contact with the capstan 10 and the pinch roller 11 is taken up by the take-up reel 6.
FIG. 3 shows a block diagram of a conventional f-v converter, and FIG. 4 shows a timing chart of signals generated from the main part thereof. Referring to FIG. 3, a reel FG signal S.sub.1 is supplied to a pulse generator 42 through an input terminal 41. A sampling pulse signal S.sub.2 is generated by the pulse generator 42 in response to every leading edge of the reel FG signal S.sub.1. The pulse generator 42 also generates a reset pulse signal S.sub.3 which is slightly delayed from the sampling pulse signal S.sub.2. The reset pulse signal S.sub.3 is supplied to a triangular wave shaper 43. A triangular wave signal S.sub.4 from the triangular wave shaper 43 is supplied to a sample/hold circuit 44. The sample/hold circuit 44 also receives the sampling pulse signal S.sub.2. The voltage of the triangular wave signal S.sub.4 is sampled-held by the sample/hold circuit 44 which then generates a signal S.sub.5. The signal S.sub.5 appears at an output terminal 45 and is supplied to the mixer 25.
In this conventional example, a constant current I.sub.S flows in the supply reel motor 7 such that a torque is generated from the supply reel motor 7 so as to generate a tape tension (counterclockwise) in a constant-speed mode. A torque .PHI..sub.S generated from the supply reel motor 7 is given as follows: EQU .PHI..sub.S =K.sub.TS .multidot.I.sub.S ( 1)
where K.sub.TS is the torque generation constant. A tape tension T.sub.S at an exit of the supply reel is given below: EQU T.sub.S =.PHI..sub.S /R.sub.S ( 2)
where R.sub.S is the radius of the tape coil wound around the supply reel. Since the torque .PHI..sub.S generated from the supply reel motor 7 is kept constant, the tape tension T.sub.S at the exit of the supply reel 5 is increased in inverse proportion to a decrease in the radius R.sub.S of the coil of the magnetic tape wound around the supply reel. When the magnetic tape passes along various posts, the tape tension is increased at a rate e.sup..mu..theta. (where .mu. is the tape friction coefficient and .theta. is the tape winding angle), so that a tape tension T.sub.CI at the entrance of the capstan 10 becomes very high. Similarly, a constant current I.sub.T flows in the tape-up reel motor 8, such that a torque .PHI..sub.T generated from the take-up reel motor 8 is given as follows: EQU .PHI..sub.T =K.sub.TT .multidot.I.sub.T ( 3)
where K.sub.TT is the torque generated from the take-up reel motor 8. A tape tension T.sub.T acting at the entrance of the take-up reel 6 is given below: EQU T.sub.T =.PHI..sub.T /R.sub.T ( 4)
where R.sub.T is the radius of the coil of the magnetic tape 2 wound around the take-up reel 6. Since the torque .PHI..sub.T generated from the take-up reel motor 8 is constant, the tape tension T.sub.T acting at the entrance of the take-up reel 6 is decreased in inverse proportion to an increase in the radius R.sub.T of the coil of the magnetic tape 2 wound around the take-up reel 6. When the magnetic tape passes along various posts, the tape tension is decreased at a rate of e.sup..mu..theta., so that a tape tension T.sub.CO at the exit of the capstan is decreased. Therefore, a difference between tensional forces at the entrance and exit of the capstan 10 is given below: EQU .DELTA.T.sub.C =T.sub.CI-T.sub.CO ( 5)
thus resulting in a great difference or change.
In this conventional example, the capstan motor for driving the capstan requires a great torque. In addition to this disadvantage, it is undesirable to change the tape tension in a constant-speed mode in accordance with the radius of the magnetic tape 2 wound around the supply and take-up reels 5 and 6. Furthermore, the f-v converter comprises an analog circuit, so that a large number of component parts is required.